you may need to assign a drive letter to the HDD. Find your "my computer" icon. This can either be on your desktop or in the start menu. Right click and click on manage.
click on disc management. Right click on the partition that does not have a drive letter and seems to be close to the correct size. there should be a way to assign a drive letter to the drive. After you do that it should show up in windows explorer. The other option would be that you need to format the drive. This is done the same as above.Oh by the way. if you format the drive/partition you will lose all data that is currently on the drive.